Damaged Speleothems and Collapsed Karst Chambers Indicate Paleoseismicity of the NE Bohemian Massif (Niedźwiedzia Cave, Poland)

Multiphase speleothem damage and passage collapse in Niedźwiedzia Cave (NE Bohemian Massif, Poland) were dated with U‐series methods, revealing five events: (1) 320–306 ka, (2) 253–236 ka, (3) 162–158 ka, (4) 132–135 ka, and (5) >21 ka. Events 1, 3 and 4 are robustly constrained, and events 2 and 5 are less certain. Although we cannot unambiguously exclude other agents (frost or gravity collapses), the most likely trigger of damage in the cave was an earthquake, which is supported by timing (the damage occurred independently from climatic conditions in cold and warm periods) and deformation style (damage to the ceiling and walls as well as the passage floor). We applied ground motion models to determine the probable seismic source size, which is most likely the Sudetic Marginal Fault ‐ one of the most pronounced tectonic structures in Central Europe. Located <20 km from the cave and with documented earthquakes of M > 6, the Sudetic Marginal Fault can produce peak ground acceleration values high enough to break speleothems. The other plausible seismic sources are faults in the Upper Nysa Kłodzka Graben located to the east and the Trzebieszowice‐Biela Fault. Although there are sparse historical data that would allow estimating linked seismic hazards, the <8 km distance between the cave and faults should suffice to destroy the speleothems. Niedźwiedzia Cave shielded environmental earthquake effects from erosion. This study shows the advantage of employing speleoseismology in moderate seismic regions, where earthquake effects are rarely preserved in the geological record.

One of the types of information indicating past processes is destruction in caves, and if the damage includes speleothems, the age of the destruction can be established. Worldwide, cave damage, especially speleothem breakage, is interpreted or even used as a proof of concept for paleoseismic studies (see the review by Becker et al., 2006). However, modeling and laboratory tests show that speleothems break with great difficulty under ground shaking; in addition, several conditions must be met, such as a significantly elongated speleothem shape (Gilli et al., 1999;Gribovszki et al., 2018) or a short distance from the epicenter . These studies and tests are based on an assumption of a prismatic cantilever beam supported at one end (Kong et al., 2008), which can be treated as a simple model of a stalactite or stalagmite. The failure process of speleothems, caused by any oscillating source, depends on the natural frequency of speleothems and the corresponding horizontal ground acceleration. These two critical parameter values are a result of speleothem size, density, elastic parameters, and tensile failure stress (e.g., Cadorin et al., 2001;Lacave et al., 2000;Pace et al., 2020;Szeidovitz et al., 2008). Therefore, an analysis of the failure process may allow inference about their coseismic causes. Despite the general resistance of speleothems to destruction, recent damage in caves caused by earthquakes has been reported (Pérez-López et al., 2009;Zhao et al., 2020). Speleoseismology is therefore based on mechanisms that are not fully understood, which in some cases leads to opposite interpretations of destruction origins within the same cave. Usually, the discussion oscillates between seismic damage and frost and ice action, such as in the Postojna Cave (Kempe, 2004(Kempe, vs. Šebela, 2008(Kempe, , 2010 or our study area, the eastern Bohemian Massif, where Bábek et al. (2015) postulated an earthquake origin, while Žak et al. (2019) stated that the caves were also affected by ice. Furthermore, other causes of speleothem deformation, such as clastic substratum movements (compaction, soil creep, gravitational mass movement or liquefaction), floods, mud and debris flows, and human or animal impacts, are also possible.
Recently discovered caves or cave sections are the most valuable for speleoseismology research because anthropogenic factors are negligible (Becker et al., 2006). Such circumstances are met in Niedźwiedzia Cave in the village of Kletno (Sudetes, SW Poland), where approximately 3 km of underground passages were discovered during 2011-2013 (Sobczyk et al., 2016). Since then, several dozen broken and fallen stalagmites, stalactites, and flowstones have been documented, the largest of which is nearly 0.6 m in diameter and ∼2 m in height. Damaged speleothems, as well as ceiling collapses, can be observed in the whole cave, where the debris thickness reaches several meters, but the cave is remodeled the most in the newly discovered sections.
This paper aims to decipher the causes of damage in Niedźwiedzia Cave. We completed the dating of broken speleothems with a spatial analysis of cave morphology with particular reference to collapses. We support our interpretation with a probable seismic intensity prediction to define a seismogenic source and to calculate the expected macroseismic intensity and peak ground acceleration (PGA) at the research site. Finally, we evaluate probable seismic effects at the site with the elastic properties of speleothems to ascertain a threshold value of failure.

Geological Setting
Niedźwiedzia Cave is located on the northern slopes of the Śnieżnik Massif (W Sudetes; Figure 1) and was formed within the Stronie Formation. The Stronie Formation comprises fine-grained Neoproterozoic metasedimentary and metavolcaniclastic rocks and mid-Cambrian-Early Ordovician amphibolites, paragneisses, mica schists (532 Ma; Mazur et al., 2015), and marbles (520-470 Ma; Jastrzębski et al., 2010). The Śnieżnik Massif constitutes the easternmost part of the Orlica-Śnieżnik Dome (OSD; Figure 1b), which is a vast geological unit formed as a result of large-scale crustal folding during the Variscan orogenesis (Chopin et al., 2012). In its central part, the OSD is divided by the Upper Nysa Kłodzka Graben (UNKG; Figure 1b), a fault-bounded N-S elongated tectonic graben that originated during the Late Cretaceous (Don, 1996). The UNKG represents the northernmost part of the Nysa-Morava Zone, a tectonically active region of the NE Bohemian Massif, which is bounded by the Western Carpathians front (Špaček et al., 2015). Miocene-Pliocene to Quaternary tectonic uplift of the Bohemian Massif activated alkaline basaltoid extrusions located to the north of the Śnieżnik Massif (Birkenmajer et al., 2002). During the Middle Pleistocene, the Scandinavian ice sheet reached the Eastern Sudetes (Badura & Przybylski, 1998), resulting in the deposition of glacial and glaciofluvial sediments within the UNKG during the Elsterian glaciation (MIS12).

Neotectonics of the Study Area
The Cenozoic geodynamic activity of the Śnieżnik Massif and the Sudetes, in general, has received far less attention than the Variscan and Permo-Mesozoic stages. The most intensive post-Variscan tectonothermal rebuilding of the OSD is linked with the end Cretaceous-Paleogene deep burial and basin inversion phase, which was revealed through apatite fission track dating results (Sobczyk et al., 2020). Its latest paroxysms and cessation of cooling commenced during the Middle Eocene (∼45 Ma); hence, Eocene-to-recent exhumation has been responsible for the last ∼1 km of erosion, for the most part, and has heavily accelerated since the Late Miocene (10 Ma). Growth of the Śnieżnik Massif topography most likely resulted from a longwave 500-1,000 m uplift realized along rejuvenated brittle structures (Sobczyk et al., 2020 and references therein). During the Neogene (Late Miocene-Pliocene?), active fault-block movements in the eastern part of the Śnieżnik Massif associated most-likely with WNW-ESE compressional regime (Sobczyk & Szczygieł, 2021) Štěpančiková et al. (2010). Present-day seismicity (yellow circles) after earthquake catalogs (https://www.usgs. gov/products/data-and-tools/real-time-data/earthquakes, and https://www.emsc-csem.org); historical earthquake locations (after Štěpančiková et al., 2010) marked as gray stars with numbers (year, epicentral intensity I 0 , n/a = data not available): (1) 1877, I 0 = n/a; (2) 1562, I 0 = 7; (3) 1786, I 0 = 4; (4) 1615, I 0 = 5; (5) 1877, n/a; (6) 1496, I 0 = 5; (7) 1895, n/a; (8) 1778, I 0 = 4; (9) 1594, n/a. White inset shows the extent of Figure 9. is no direct evidence of this phase recorded in the deeply incised V-shaped Kleśnica valley in the area of Niedźwiedzia Cave. As inferred from geological map analysis, the WNW-ESE-trending Igliczna-Kletno fault ( Figure 1) displaced the eastern UNKG boundary by approximately 1 km. This dextral fault postdates the UNKG formation and guides the W-E part of the Kleśnica valley below Niedźwiedzia Cave. The UNKG is a northern part of the seismically active Nysa-Morava Zone (Roštínský et al., 2020;Špaček et al., 2015), wherein, located approximately 20 km to the NE of the research area, the Sudetic Marginal Fault (Badura et al., 2007) is one of the most prominent tectonic features. The pre-Neogene activity of the SMF was studied by Danišík et al. (2012), whereas Štěpančíková et al. (2010) discussed its latest Pleistocene-Holocene history, suggesting that it was active ca. 11 ka BP as a reverse fault, and it then reactivated as a normal fault during the early Holocene. The Middle and Late Pleistocene uplift along the SMF is assumed to be a combination of glacioisostatic rebound and tectonics, with total vertical movements in the range between 20 and 80 m (Krzyszkowski & Pijet, 1993;Štěpančíková & Stemberk, 2016;Štěpančíková et al., 2008). Importantly, recent GNSS measurements combined with geological, geomorphological and geophysical data show significant, mainly dextral movements along the SMF (Roštínský et al., 2020). Within a range of 30-40 km from Niedźwiedzia Cave, one of the most seismically active structures is the Hronov-Pořiči Fault Zone (Sobczyk & Szczygieł, 2021), the easternmost part of the important central European tectonic structure-the Elbe Fault system (Málek et al., 2008). The Elbe Fault system contains mostly dextral NW-SE and NNW-SSEstriking faults and is suspected to be responsible for the most intense historical earthquakes in the NE Bohemian Massif (Špaček et al., 2006).

Cave Morphology and Geology
The artificial entrance to Niedźwiedzia Cave is located in an old quarry and is 800 m a.s.l. and 10 m above the bottom of the fault-guided Kleśnica valley. The cave is 4,081 m long with a vertical extension of 118 m (−28,70;+89,60;Kostka, 2014). The cave is hosted by an elongated lens of strongly folded Lower Cambrian marbles intercalated with mica schists.
Two morphologically different parts can be distinguished within the cave. The northern part extends from the entrance to the Mastodont Chamber and constitutes a maze network (sensu Palmer, 2007) with NNW-SSE-and ESE-ENE-dominated conduit directions ( Figure 2). Passages have mostly lenticular cross-sections that are elongated diagonally, and oval tubes are also common. The southern part, including the Mastodont Chamber and southern passages, is parallel to the Kleśnica valley. Large volume N-S-running galleries are irregular in cross-sections due to breakdown remodeling. Nevertheless, relatively similar width and height dimensions suggest that the original morphology was more tubular than canyon-or fissure-like. Additionally, solution features are damaged, and only a few solution notch relicts have been locally documented (Figures 2e and 2f). Vertically, three cave levels can be distinguished (Figure 2b).

Cave Geomorphological Mapping and Sampling
Morphological observations focused on recording modifications of original karst morphology by breakdown, which is the third most commonly occurring morphological type in caves in general (Ford & Williams, 2007). We distinguished four grades of breakdown influence on cave passage morphology: from no influence (1), through passages with karst morphology that are only locally modified (2), through strongly modified conduits where only relict karst morphology is preserved (single notches or rills) (3), to passages (4) where the collapse processes completely obliterate karstic morphology (Figure 2a).
Breakdown deposit thickness has been estimated in two ways. In a few places (e.g., the Mastodont Chamber), unmapped passages are located between boulders that do not reach the bedrock floor given the deposit's minimum thickness. The second possibility is to check the vertical distance between an uppermost point on a collapsed pile of rocks and the lowermost solution notch truncated by this collapse.
Moreover, the destruction of speleothems has also been evaluated. Speleothem abundance in Niedźwiedzia Cave results in a proportionately large number of destroyed speleothems distributed in the whole cave, especially in the breakdown zones. Therefore, only places where the destruction is significant have been SZCZYGIEŁ ET AL.

10.1029/2020TC006459
marked on the plan, that is, localities that can be described as "speleothem cemeteries," where broken and fallen speleothems cover the entire floor or places where there are at least a few damaged speleothems in one location and are a single but sizable feature. Furthermore, for 50 broken speleothems, we acquired geometric measurements, including length (minimum) and failure plane diameter, to investigate the relationship between speleothem dimensions and the horizontal ground acceleration required to damage speleothems (Ferranti et al., 2019;Gribovszki et al., 2018; To date a speleothem damage event, sampling sites have been chosen based on the methodology developed by Forti and Postpischl (1984), Postpischl et al. (1991), and Kagan et al. (2005). If possible, pre and postevent samples were taken from broken or fallen and regrown or enveloped speleothems, respectively. Since there is a plan to provide tourist access to parts of the Mastodont Chamber, we had to limit sampling activity in places where the trail is planned, including sample collection from the tips of stalagmites. At one site (MAS2), we also collected samples from a nondestroyed stalagmite, which was deposited on a collapsed boulder (Figure 2d), to test the possible correlation between collapse and fall of the speleothems from other sites. Wherein it did not affect the aesthetic value of the cave, we sampled the whole speleothem; if the site followed the planned tourist route or the speleothem was too large, a core was drilled.

Th-U-Series Dating
Speleothem samples were analyzed by standard 230 Th/ 234 U disequilibrium techniques (e.g., Ivanovich et al., 1992). U-series dating, including chemical separation of uranium and thorium by the chromatographic method with TRU resin (Hellstrom, 2003), was performed at the U-series Laboratory of the Institute of Geological Sciences of the Polish Academy of Sciences in Warsaw. For each analysis, 0.1-0.5 g of calcite was extracted from places with no visible detrital admixtures or porosity using a dental drill. Mixtures of 233 U-236 U-229 Th, calibrated by uraninite analysis in secular equilibrium, were used for a chemical procedure and isotopic fractionation control. The isotopic compositions of U and Th were measured at the Institute of Geology of the Czech Academy of Sciences in Prague using a double-focusing sector-field ICP mass analyzer (Element 2, Thermo Finnigan MAT; Thermo Fisher Scientific, Waltham, MA, USA). The measurement results were corrected for counting background and chemical blanks. The internal standard sample and blank sample were simultaneously prepared for series of studied samples and were used for necessary corrections and quality control.
U-series ages were calculated iteratively from 230 Th/ 234 U and 234 U/ 238 U activity ratios, and the results are presented with an error limit of two standard deviations (Table 1). The decay constants of Jaffey et al. (1971) for 238 U, Cheng et al. (2013) for 234 U and 230 Th, and Holden (1990) for 232 Th were applied. Age errors do not include uncertainties related to the decay constants. Corrected ages were adjusted for detrital contamination indicated by the presence of 232 Th using the typical silicate activity ratio 230 Th/ 232 Th of 0.83 (±0.42) derived from the 232 Th/ 238 U activity ratio of 1.21 (±0.6), 230 Th/ 238 U activity ratio of 1.0 (±0.1) and 234 U/ 238 U activity ratio of 1.0 (±0.1; cf. Cruz et al., 2005). The initial value of the 234 U/ 238 U activity ratio (Table 1) was calculated based on the corrected activity ratio and the sample age.

Ground Motion Relations
The relative range of earthquake hazards for a particular site depends greatly on the decrease in the ground motion amplitude (or intensity) with distance from all surrounding potential seismic sources, event size, and type of acting mechanism. The ground motion relation generally depends on the depth of focus, earthquake mechanism and geological conditions along the path (Schenková et al., 1981). Therefore, to discuss probable seismic effects in the caves and their vicinities, we used the attenuation curves presented previously for the research area (Procházková et al., 1990;Schenková et al., 1981;Schenk et al., 1989Schenk et al., , 2000   2001). Two linear branches approximate the individual attenuation curves. The first branch corresponds to the epicentral area (the extensions of the maximum intensity grade zone) of radius r 0 where very small or no intensity decrease occurs. In the study area, it reaches 8.3 km (Schenková et al., 1981). The second branch reflects the simplified attenuation process. The study area can be expressed by a simple formula (Schenková et al., 1981, see the coefficient for the 13th zone): where ΔI is the decrease in intensity scale I (in terms of the MSK scale) with epicentral distance r, and I 0 is the intensity in the epicenter.
The intensity in the epicenter of the reliable magnitude, M, can be estimated according to the scaling relation (Schenk et al., 2000): where I 0 is the intensity of the epicenter in terms of the MSK scale. We used these equations to estimate (or to predict) the expected intensities ranging with distance from the seismogenic source region. Notably, this approach is a steady-state approximation because we assumed that isoseismal spread is regularly distributed around the epicenter and that the local influence of discontinuities is omitted. However, the 13th zone selected by Schenková et al. (1981) is relatively small, and it can be assumed that it meets the criteria for seismic zonation (Sitharam et al., 2018). To make the results more convenient in interpretation, the intensity attenuation relations were calibrated in terms of peak ground acceleration (Schenk et al., 2000): where PGA is the peak ground acceleration (cm/s 2 ), and I is the intensity on the MSK scale.

Speleothem Failure Parameters and Criteria
The failure criteria resulting in speleothem damage were evaluated on a prismatic cantilever beam supported at one end. The parameters describing these criteria are the speleothem natural frequency and the corresponding critical horizontal ground acceleration, which depend on elastic properties of the speleothem and the following geometric dimensions: height, H, and diameter, D, measured in situ (Cadorin et al., 2001;Ferranti et al., 2019;Lacave et al., 2000;Pace et al., 2020;Szeidovitz et al., 2008). The horizontal ground acceleration is the limit at which speleothems break. The natural frequency indicates that the failure process is more likely to occur if the natural frequency (or resonance frequency) is in accordance with the seismic wave frequency affecting the speleothem. If both frequencies match, the speleothem resonates, enhancing the oscillation effect, which may lead to damage.
The values of both failure parameters depend on elastic properties that can be assessed in laboratory tests in which the following parameters are estimated: P-and S-wave velocities, V p and V s , respectively, using an acoustic measurement system (AMS; see details in Szczygieł et al., 2020); density, ρ (from the measured mass and volume); Young's modulus, E; and tensile stress, σ t . Static horizontal ground acceleration (a g in m/s 2 ), required for speleothem breakage, can be calculated as follows : where H and D are the speleothem height and diameter, respectively.
The natural frequency (eigenfrequency) for a given speleothem (stalactite or stalagmite) using Young's modulus, E, can be expressed as : where α n (α 1 = 0.560, α 2 = 3.507, α 3 = 9.817…) results from the Euler-Bernoulli equation for fundamental and higher harmonic modes (Kong et al., 2008;Lacave et al., 2000). The AMS tests were carried out at the Institute of Earth Sciences, University of Silesia, Sosnowiec, Poland. The failure stress was estimated based on the uniaxial compressive stress test carried out in the external laboratory of AP GEOTECHNIKA, Katowice, Poland, using a universal testing machine. The strength tests were conducted for 0.05 × 0.05 m (height/ dimension, h/d = 1) rock samples. The sample size does not meet the ISRM requirement (equal to 2); therefore, the uniaxial compressive stress, σ c , was corrected according to the formula of Peng and Zhang (2007): An approximation of the uniaxial tensile strength was as follows (Peng & Zhang, 2007): where F is the yield load in lbf, l is the sample width in mm, and I s50 is the calculated standard index in psi, which is σ c /21. In this case, the yield load was approximated from the compressive stress test multiplied by the sample surface and transformed from N to psi units. Next, the calculated tensile stress was used in the acceleration calculation (Equation 4).

Sampling Sites
We collected 15 speleothem samples from six sites in Niedźwiedzia Cave. Three samples were taken from the Mastodont Chamber. Sample JN-JS-1 is a stalactite approximately 1 m wide that fell into breakdown debris and was enveloped by younger flowstone (Figure 3a). The exact place from which the flowstone fell off is impossible to point out unequivocally because the wall above is fully covered with speleothems. Below the fallen flowstone, an older stalagmite was present. The contact of pre and postevent layers is visible in the drill core. In the southern part of the Mastodont Chamber, the stalactite population formed along the ceiling fissure has been broken. Younger stalagmites have covered fallen stalactites. Sample MAS1 is a fallen stalactite (preevent layer) enveloped with young calcite (postevent layer), which forms a small positive stalagmite-like form at the top and a thick stalactite underneath (Figures 3d and 4c). From the north, the Mastodont Chamber is closed by a huge boulder measuring ca. 10 × 5 × ∼6 m, which fell off of the ceiling. The western wall above the boulder is a fault plane; however, since slickensides indicate oblique-dextral movement, there is no collapse effect. Stalagmites that have grown on the boulder were sampled (MAS2; Figure 2c) to reveal their older layer age, possibly indicating a minimum age of the collapse episode (Figure 4e).
Farther to the south of the Mastodont Chamber, where three horizontal passages are superimposed, a zone of large (up to 90 cm wide) fallen stalactites, partly covered by younger flowstone and stalagmites, is located at the uppermost level of the Balkonik site ( Figure 2). One of the stalagmites (sample BA1) was taken with a portion of a fallen stalactite (Figures 3e and 4b).
The Humbaki chamber is the second largest chamber in the cave, wherein broken-down boulders cover the entire floor. Walls here usually form a system of flat structural surfaces without any dissolution features. Stalagmites are quite common on boulders, and they often cover debris without breaking signatures, and most are still growing. These features suggest that they are relatively young, at least much younger than the collapse. The age relation with the collapse event is problematic to estimate, so we did not date them. Nevertheless, in the NE part of the Humbaki Chamber, debris leans on a large 60 cm-wide broken stalagmite. In addition, a few smaller but still large (120 × 25 cm) stalagmites partly covered by breakdown debris exist in the surrounding area. Three of them were sampled (Figure 3b). HU1 is a 15 cm-long core drilled to reach the oldest part of the regrown stalagmite. HU3 is the top of the broken stalagmite covered by breakdown debris. HU2 and HU4 were sampled from a smaller broken stalagmite. HU2 is a core from the top of an older broken stalagmite, and HU4 is a core drilled in the regrown stalagmite that contacts the broken and regrown stalagmites.
Several broken stalagmites and columns are located in dissolution notches on the passage curve at the Zaułek site (Figure 3f). Importantly, these speleothems were deposited directly on bare marble bedrock. Core sample ZAU 3 was drilled to reach the oldest part of the regrown stalagmite. ZAU 1 is the overturned, fallen stalagmite in the same location as the regrown stalagmite and is a whole regrown stalagmite and a portion of the older stalagmite (Figure 4d). At the Zaułek site, a few more damaged speleothems occur, including a shifted column.
In the Galeria Jerzy passage, two sites were sampled. In one of them, solution notches covered with flowstone (GJ2b) have been cut by collapse. In the second site, there are broken (GJ4b-stalagmite top) and regrown stalagmites (GJ4a-drilled core), which were sampled (Figure 3b).
The Kutaśnik Chamber is the southernmost chamber and is filled with a mixture of alluvial fine-grained clastic deposits and collapsed boulders. The upper part of the chamber is covered by several dozen (perhaps more than one hundred) stalagmites, many of which are broken, tilted, cracked, and have a curved growth axis. On the edge of this rock pile, two stalagmites were collected. KUT1 is a fallen stalagmite with a curved growth axis cemented to the ground by younger calcite. The preevent sample represents the youngest stalagmite layer, and the postevent time can be estimated from the calcite that cemented the speleothems to the schists (Figure 4a). KUT3 is stalagmite on a boulder tilted by the younger breakdown. On the tilted stalagmite, in its lower, basal part, a new stalagmite is growing. This L-shaped form is naturally cracked and slightly detached from the base by a 2-3 mm fissure (Figures 3c and 4f).

Distribution of the Breakdown and Damaged Speleothems
The collapsed sites in Niedźwiedzia Cave are sporadic in the lower level, common in the middle level, and dominant in the upper level ( Figure 2b). The observed effects could result from a progressive vertical change in cave passage maturity as a function of exposure time to breakdown processes after its abandonment by the stream. The transformation degree and the thickness of debris roughly estimated at 30 m ( Figure 2b) south of the Mastodont Chamber are so substantial that additional factors, in addition to time, should be considered. Specifically, the speleothems in less broken-down parts of the cave were also destroyed. All collapses were detached along preexisting structures, mostly foliations and joints, and the largest collapse in the Mastodont Chamber was detached along a fault plane (Figure 2c and 2d); however, we did not recognize a particularly favorable structure for collapse development.
Speleothem damage is most common at the highest level, which includes the location of the collapse. The occurrence of damaged speleothems is noticeable in almost the whole northern part of the cave, and the significantly affected sites are marked in Figure 2a. There is no form that has been destroyed in particular. The damage affected the whole spectrum of speleothems: soda straws, 60 cm-wide stalagmites, and a 90 cm-wide stalactite that is semiconnected with the wall.

Th-U-Series Dating
The U-series dating results are presented in Table 1. An isotope of 232 Th indicates the sample's possible contamination by thorium and uranium from the detrital source. For mass spectrometry data, the value of the 230 Th/ 232 Th activity ratio, which is equal to 200-300, was considered a border value for clean samples (Hellstrom, 2006). Calculated ages for samples with 230 Th/ 232 Th activity ratios below 200 were corrected for detrital contamination. The correction results agree with the uncorrected values in the 2σ range.

Several years of research conducted in Niedźwiedzia
Cave confirmed that precise dating of speleothems in this cave is difficult. The main problem is that very low concentrations of 238 U prevent high-precision U-Th dating. This problem was first discussed with early results of the U-Th dating of Vistulian-Holocene speleothems from Niedźwiedzia Cave (Hercman et al., 1995). Recently, several attempts to precisely U-Th date Holocene stalagmites collected as a part of paleoclimatic studies were also unsuccessful for this locality. Typical dating errors with Holocene samples were between 190 and 4065 years, with numerous stratigraphic inversions (Lechleitner et al., 2016). Other problems when dating speleothems using the U-Th method are detrital contamination and the high porosity of most of the samples. These problems were also revealed despite several attempts to date with a larger sample population (>60); finally, we attained only 26 reliable analytical results (Table 1).

Ground Motion Relations-Estimates of PGA and MSK
Estimation of the ground motion relation requires information about paleoseismic event size and intensity. Event magnitude was estimated using previous studies concerning seismological and paleoseismological investigations that used the empirical scaling relations between magnitude and parameters representing the faulted area. They include fault rupture length, surface rupture length, downdip fault rupture width, maximum displacement, average displacement, and many others (e.g., Wells & Coppersmith, 1994;Leonard, 2010Leonard, , 2014. To date, the greatest value corresponds to the paleoseismic event of M6.3, which was reported from the Sudetic Marginal Fault (Štěpančíková et al., 2010) that is located ca. 20 km from Niedźwiedzia Cave. The reported M6.3 magnitude was derived from vertical offset inferred from the height of the fault scarp beneath the colluvial wedge (0.3 m) and is based on an empirical relationship-magnitude versus maximum vertical displacement-for reverse faults (Wells & Coppersmith, 1994). However, using formulas proposed by Leonard (2010Leonard ( , 2014, the discussed event could have had an even larger magnitude exceeding M7.0 (Table 2). Following Leonard's (2010) proposition, we assumed that the average displacement is half of the maximum displacement, and we used a scaling relation. By applying the abovementioned procedure, we strove to minimize the general limitation of Leonard's (2014) scaling relation, which concerned only two types of intraplate faults (strike-slip and dip-slip), as well as the related magnitude with average displacement. Notably, none of the authors provided the average displacement; thus, we assumed half of the value of the maximum displacement.
Simulating possible scenarios, we assumed four different magnitude thresholds: M5.7, M6.4, M7.1, and M7.6, and the corresponding intensities in epicenter I 0 were 8, 9, 10, and 10.8, respectively. Next, the intensity attenuation relation (Equation 1) allowed us to estimate the decrease in the intensities with distance for the chosen I 0 . To facilitate further interpretation, the intensities were recalibrated to PGA according to Equation 3. The calculation results are shown in Figure 5, and raw data are available in the supplementary materials.

Speleothem Failure Assessment
We measured the height and diameter of 50 broken speleothems observed in Niedźwiedzia Cave, including 36 stalagmites and 14 stalactites ( Figure 6). The distributions of the measured heights and diameters are shown in Figures 6a and 6b, respectively. In addition, we calculated the aspect ratio (AR = H/D = height/diameter) for the studied speleothems following the method of The elastic properties, that is, P-and S-wave velocities, density, and compressive stress, were tested for four selected speleothems whose size allowed the preparation of the appropriate samples (BA1, KUT3, GJ4B, and MAS1). The velocities and dynamic Young's moduli were obtained from wave travel times measured by an AMS (Table 3). The compressive stress was measured for two samples (BA1 and KUT3; Table 3). The size of the broken speleothems and sample preparation for these two types of tests made it impossible to perform the same tests on each sample. Therefore, dealing with these scant data, we assumed that average values were representative of all speleothems in this study. Next, the tensile stress, natural frequencies, and horizontal ground acceleration values were calculated according to formulas (5) and (4).
To approximate the possible failure criteria, the average values of the elastic parameters were calculated for each of the speleothem samples. The results of the natural frequency versus the H/D ratio and versus the height are shown in Figures 6d and 6f, respectively. Similarly, the estimated horizontal accelerations are shown in Figures 6e and 6g. The distribution of stalagmites and stalactites in Figure 6 suggests that both types of speleothems are clustered, that is, the relation of the natural frequency to the H/D ratio indicates that stalactites are characterized by high frequencies and have a larger H/D ratio than stalagmites SZCZYGIEŁ ET AL.
10.1029/2020TC006459 12 of 25  ( Figure 6d). Similar observations are true for the horizontal acceleration ( Figure 6e). Moreover, f 0 and a g depend more strongly on height (Figures 6f and 6g) than on the H/D ratio.

Timing of Damage in Niedźwiedzia Cave
The fundamental mechanism of time constraint delimitation in speleoseismology, as in "classic"-trenchbased paleoseismology-is bracketing (Kagan et al., 2017;McCalpin, 1996). Although we sampled the sites according to rigorous methodology, theoretically, to determine the minimum and maximum damage ages, some samples were not suitable for dating due to objective reasons (these could not be assessed during fieldwork).
The oldest damage was recorded in the Humbaki Chamber, where the age of the tip of the large broken stalagmite (HU3) was determined to be   23 20 320 ka. The regrown stalagmite (HU1) was analyzed by a 15 cm core, which limits the possibilities of reconstruction of the internal structure of the speleothem. At the bottom of the core, a fracture was recognized. The fracture walls were covered with a skinny layer of detrital material, on which columnar calcite crystals grew perpendicular to the open fissure walls and demonstrated angular unconformity with older, prefractured thin-layered calcite (Figures 7c and 7d). The columnar calcite crystals constituted the oldest postfracturing layer, dated at 253 ka. Within the Gj4a sample, representing postfracturing deposition, in the basal part of the core, we identified an unconformity highlighted by a thin layer of detrital material, wherein the oldest layer suitable for dating revealed a date of 147 ± 4 ka ( Figure 7a). Notably, significant damage was also found at the other sites, where postfracturing samples yielded ages of 236 ± 6 ka (JN-JS-1; Figure 7b) and 198 ± 7 ka (BA1; Figures 3f and 4b), respectively. If we assume that both failures coexisted, we can speculate that event 2 might have occurred between 253 and 236 ka.
Event 3 is the best documented event, mainly because it was possible to determine the coherent ages of both brackets at several sites located away from each other. Sample KUT3 from the Kutaśnik Chamber yielded the youngest prefracturing age, 159 and 153 ± 44 ka. Moreover, from the same passage, the MAS2 stalagmite postdates a huge collapse, whose oldest layer is 158 ± 3 ka. It allows us to limit event 3 to the time range of 162-158 ka (Figure 8).
Event 4 occurred between ca. 135 and 130 ka (Figure 8), as indicated by two samples. First, the fallen stalagmite in the Humbaki chamber had a tip (HU2) that dated to 133 ± 5 ka. The second is the KUT1 stalagmite, the youngest layer of which was dated to   8 6 132 ka. The postfracturing layer age was estimated as 135 ± 5 ka. Considering uncertainty, this result suggests that postfracturing calcite deposition began immediately after stalagmite damage and that event 4 occurred from approximately 140-130 ka.
Younger samples from the Zaułek site (ZAU1 and ZAU3) provided only postevent information, while the prefracturing layers had an open system, that was unsuitable for dating. However, in both cases, the postfracturing ages are tightly clustered, that is, 21 ± 2 (ZAU3/A1) and 17.4 ± 0.5 (ZAU1), which suggests that their damage may have been related to Weichselian event 5 (Figure 8) (not later than MIS 2). Nevertheless, this event is poorly constrained by U-Th dating and thus remains speculative.

Discussion-Cave Damage Causes
In speleoseismology, the first step is to consider all possible nonseismic causes and exclude them or acknowledge them as alternative interpretations (cf. Becker et al., 2006). In Niedźwiedzia Cave, large Pleistocene mammal bones occur in the sediment profiles at the entrance (Baca et al., 2014) and not in deeper caverns where speleothem samples were collected, so speleothem destruction should not be connected with large mammals activity. All samples in this study were retrieved from speleothems growing on solid rock or large boulders; hence, the interaction of any kind between unconsolidated fine sediment and speleothems, such as subsidence and underlying clastic deposit movements due to drying, flux, or liquefaction, can be excluded. There are also no signs of mass movements at the surface and displacements in the cave, which allows the exclusion of this cause as well. Three determinants require a more in-depth discussion: gravity collapse, cryogenic fracturing, and seismic fracturing.

Collapse-Effect or Trigger of Tremors?
Breakdown morphology is the third most common type in caves and originates as a response to remodeling of the original phreatic or vadose forms (Ford & Williams, 2007). Physically, a cavern is affected by the stress tensor pattern known as the tension dome, a model adopted for caves based on mine studies (Davis, 1951). When a conduit's shape is not in an equilibrium state with the rock mass, the cave passage shape transforms into an arch-like cross-section via breakdowns. The main difference is in the tension on the floor, which is much smaller in caves than in mines (Criss et al., 2008;Ford & Williams, 2007). In mines, the floor is pushed up, while in caves, it is stable. SZCZYGIEŁ ET AL.  Wells and Coppersmith (1994) 4.1 9.9 4.2 6.9 Leonard (2014) Dip-slip (Normal) 15.0 15.3 6.2 6.3 Wells and Coppersmith (1994) 3.9 11.9 4.2 7.6 Leonard (2014) a D max -maximum displacement, D av = D max /2 average displacement.

Table 2
Magnitude and Rupture Length Comparison According to Wells and Coppersmith (1994) and Leonard (2014)   The causes of the collapse are mainly related to the passage's adjustment to the natural stresses present in the rock mass (E. L. White and White, 1969). Most of the collapses result from losing support, for example, water-table drops, periodic flooding, and wall undercutting due to dissolution or mechanical erosion (E. L. White and White, 1969). In an entrance part of a cave or in ice caves, walls can also be fractionated by frost action (Oberender & Plan, 2015). In specific cases, boulders are detached by mineral (mostly gypsum) crystallization in fissures (W. B. White and White, 2003). Earthquakes can also induce collapses; notably, a small-magnitude event is enough to trigger a collapse, for example, the M4.8 Mula (Spain) earthquake in 1999 induced ceiling collapses in Benis Cave approximately 30 km away (Pérez-López et al., 2009). Even with a longer hypocentral distance (80 km), the same magnitude event caused a collapse in the Han-sur-Lesse Cave, Belgium (Camelbeeck et al., 2018). Massive collapses can also be considered a trigger of ground-motion vibrations. Analogous to surface rock falls, block impacts can produce high-frequency waves in the cave, which are quickly attenuated (Dammeier et al., 2011). Due to the geometry of speleothems, high-frequency SZCZYGIEŁ ET AL.

10.1029/2020TC006459
16 of 25 events are favorable for speleothem destruction (Bottelin et al., 2020;. Thus, within the short distance from the block fall (one dozen to hundreds of meters), speleothem failure due to seismic wave propagation is plausible. Such a massive block fall in the Mastodont Chamber is linked with event 3 (Figures 2c and 2d). Ceiling collapse can be an effect of many causes; however, floor deformation occurs less often and is difficult to explain. In Niedźwiedzia Cave, there are many places where solution notches are cut down, especially in parts located between the Mastodont and Kutaśnik Chambers (Figures 2e and  2f). Incision into older forms is a common phenomenon in caves, and in Niedźwiedzia Cave, two generations of dissolution morphology (subsequent notches and scallops; Figure 2) are observed. However, lower (younger) notches are cut. Walls below the notches are structural planes (foliation, faults, or fractures) with no corrosion signs, and the floor is covered with debris. Since the floor in a cave is stable, during an earthquake, witnesses have felt vibration in the floor (cf. Becker et al., 2006), which can be explained by tunnel waves, that is, surface waves (Rayleigh and Love waves) propagating along cave floors, ceiling and walls . High-frequency seismic waves evoke micromovements along fractures in bedrock, destabilize them and force collapse. Hence, the collapses in Niedźwiedzia Cave, which are characteristic not only of the ceiling and walls but also of the floor, may suggest that they are earthquake-triggered. Importantly, the horizontal acceleration required to break most of the speleothems in Niedźwiedzia Cave (see Figure 6) is comparable to that produced by an earthquake, as they are too high to be produced by block collapse, which does not exceed 1 m/s 2 (Saló et al., 2018). Consequently, we interpret that at least some of the collapses in Niedźwiedzia Cave are earthquake-induced.

Climate Versus Tectonics
During the Pleistocene, the Sudetes were actively remodeled under periglacial conditions, dominating the Scandinavian Ice Sheet foreland (Traczyk & Migoń, 2000). Based on the vestigial geological record for the Quaternary glaciations for the Sudetes, three different till horizons are commonly reported, which are traditionally assigned to continental ice cap oscillations: two horizons linked with the MIS 12 ice sheet (Elsterian glaciation) and the last horizon deposited during the MIS 8 glaciation (Saalian) in the Fore-Sudetic Block (Badura & Przybylski, 1998). Since the oldest in-cave damage was dated at MIS 9, two Pleistocene glaciations can be considered destruction precursors: Saalian (MIS 8-6) and Weichselian (MIS 2-4). The Saalian ice sheet during its maximum extent (Odranian stage -MIS 8 equivalent) was probably too thin (<200 m) to cross the mountain front and stopped at the Fore-Sudetic Block (Badura & Przybylski, 1998). During MIS 6 (Wartanian stage), the ice sheet front stopped approximately 100 km north of the Śnieżnik Massif. Weichselian glaciations had an even smaller range terminating approximately 150 km north of the Śnieżnik Massif (Marks, 2004). At the same time, traces of periglacial processes, including permafrost activity, were found in the Sudetes (Traczyk & Migoń, 2000) and to the south in Moravia (Czechia; Vandenberghe et al., 2014). According to Hall and Migoń (2010), erosion beneath the cold-based ice patches in the Sudetes during MIS 12 and MIS 8 was minimal, suggesting a relatively short period of their existence.
Additionally   Table 3 Results of the Speleothems Elastic Properties Tests in Central Moravia (Czechia) during the Pleistocene (south of the study area). Concerning the present-day morphology, the in-cave sampling sites are located 60-115 m from the slope surface, measured vertically, and from 75 to more than 150 m measured horizontally. These distances are not sufficient to unambiguously exclude the influence of permafrost on speleothem breakage during the Pleistocene. However, U-Th-documented speleothem growth during cold periods implies the long-term presence of unfrozen percolating water inside the cave (Figure 8), which would contradict deep (>50 m) permafrost existence. Notably, one of the known problems with testing the "creeping ice theory" (e.g., Kempe, 2004;Žák et al., 2019) is that ice cover inside caves grows for hundreds of years (Hercman et al., 2010), and it can disappear abruptly in just a few decades. For this reason, when dating old speleothems (>150 ka), short-term episodes of ice growth and disappearance can be unrecorded. Hence, we cannot unambiguously exclude the presence of ice and permafrost in Niedźwiedzia Cave. However, the time sequence of U-Th established events more likely suggests a climate-independent origin of speleothem destruction.
Event 1 occurred in a warm period (MIS 9); hence, in this case, cryogenic processes as triggering factors are excluded. Events 2, 3, and 4 occurred during cold periods (MIS 8 and 6). However, in several places in the cave, speleothems grew during MIS 8 and 6 (BA1; Hu4-Hu2; KUT1, MAS2). Because the process of speleothem growth indicates the presence of dripping water in the cave, it does not occur if the temperature in the cave is permanently below the freezing point. However, there are several examples from Polish territories of continuous growth of speleothems during glacial periods, even in glaciated (alpine-type) mountains  or places located dozens of kilometers from the ice sheet front . Accordingly, results from the Gj2b site suggest that permafrost did not reach the Niedźwiedzia Cave interior. The recorded Gj2b age indicates speleothem growth during a glacial period associated with an ice sheet front located approximately one dozen km away from the cave at this time ( Figure 8). Consequently, if during MIS 8, which was characterized by harsh climate conditions, a positive temperature prevailed in the cave, it seems less likely that in MIS 6, when the ice sheet was approximately 100 km from the Śnieżnik Massif, ice could have existed in the cave. Nevertheless, considering the lack of evidence for the presence of ice, permafrost, or any other cryogenic features in Niedźwiedzia Cave thus far, we assume that the three collapse events discussed were controlled by one of the two remaining factors, that is, gravity or earthquakes.

Possible Seismic Source
Seismic activity in the Sudetes is low to moderate but still noticeable (Schenk et al., 2001). Based on the tectonic framework and suggested seismogenic zones in the vicinity of the Śnieżnik Massif, we used a distance threshold of 50 km, beyond which we deem the effect of seismogenic sources negligible for coseismic damage in Niedźwiedzia Cave. Within this range, we consider three fault zones as the most favorable seismic sources that could have induced damage in Niedźwiedzia Cave: the Hronov-Pořiči Fault Zone (HPFZ), the Nysa-Morava Zone (including UNKG), and the Sudetic Marginal Fault (Figure 9).
From all possible seismic sources, the HPFZ represents the most active zone, considering both recent and historical seismicity, reaching I 0 =7 (Špaček et al., 2006;Zedník et al., 2001). However, at a 40 km distance, moderate earthquake seismic waves would attenuate below PGA<2 m/s 2 , which is not enough to cause most of the cave's recorded damage according to our calculations ( Figure 6). An exception could be the Trzebieszowice-Biela Fault Zone (Figure 9), an eastern extension of the HPFZ guiding the Biała Lądecka River valley, with associated crustal weakness zones employed by Mio-Pliocene volcanism (Birkenmajer et al., 2002). This fault is 12-20 km from the cave. One of the strongest historical earthquakes reported in the city of Kłodzko in 1562 (I 0 = 7; Pagaczewski, 1972) might have been related to this fault. At such a relatively short distance, the attenuated PGA amplitudes might reach as much as 4 m/s 2 (Figure 5b), which provides evidence that most of the speleothems could have already been destroyed with a moderate Pleistocene earthquake ( Figure 6g).
The Nysa-Morava Zone is a region where a few faults or seismogenic zones might be considered seismic sources. The nearest seismogenic zones are in the upper part of the Morava valley, which is only ∼8-12 km from Niedźwiedzia Cave; however, detected earthquakes barely exceed M1.0 (see zone 4 in Špaček et al., 2006). Morphologically, the eastern boundary of the UNKG is the most prominent and contains the Wilkanów and Krowiarki faults ( Figure 9). Additionally, the Igliczna-Kletno Fault deserves attention. It is a lower-order structure parallel to the Krowiarki Fault, which obliquely cuts the Kleśnica valley ( Figure 9) and hosts the northern part of Niedźwiedzia Cave. Geodynamic measurements have been taken at two sites along the WSW-ENE-trending fault ( Figure 2) since 1985 with a 3D crack gauge TM-71 system (Kontny et al., 2005;Mąkolski et al., 2008;Stemberk et al., 2010). Over 10 years, records indicate that the fault's response to earthquakes in the surrounding region is sporadic with signatures of major earthquakes in the neighboring Carpathians, or even far-triggered earthquakes in Turkey or Sumatra (Mąkolski et al., 2008;Stemberk et al., 2010). Unfortunately, there are no historical or prehistorical earthquake data from the UNKG, allowing us to estimate its possible impact on the cave. However, the mutual proximity of the UNKG and the cave reduces the attenuation effect ( Figure 5) and simultaneously increases the chances of destructive PGA amplitudes.
In particular, the Igliczna-Kletno Fault could provoke destructive incidents, as there would be no attenuation effect, and even a low-magnitude earthquake might produce the threshold motions needed for damage.  Figure 2a and Table 1; H-Holocene.
The Sudetic Marginal Fault (SMF) bounds the Orlica-Śnieżnik Dome (OSD) from the NE-located Fore-Sudetic Block. This fault represents an older, Variscan, low-angle Riedel-type shear structure (Aleksandrowski et al., 1997), which, according to thermochronological data, has repeatedly been active as a normal-reverse-normal fault since at least the Late Cretaceous (95-80 Ma) and has exhumed less than ∼1.5 km (Danišík et al., 2012). Although the historical earthquakes along the SMF reached I 0 = 5, paleoearthquakes have been estimated to have occurred up to M6.3 (I 0 ∼8.9) as an effect of OSD thrusting toward the northeast. Importantly, the buried fault rupture is located ca. 18 km from the cave and has produced PGA in the vicinity of the cave of ca. 3 m/s 2 with a moderate earthquake, and as much as 6-11 m/s 2 if M > 7 ( Figure 5). This is more than enough to cause significant speleothem damage ( Figure 6).
The abundance of published neotectonic research of the SMF and adjacent areas (see Różycka & Migoń, 2017, and references therein) is based mostly on topographic premises with rather scant support in the geological record. Performed morphological analyses allowed us to estimate the range of vertical movements SZCZYGIEŁ ET AL.
10.1029/2020TC006459 20 of 25 Faults are compiled by the authors based on Don et al. (2003) and Badura and Rauch (2014). along the SMF to between 2 and 80 m during the Middle and Late Pleistocene, depending on a particular fault segment (Badura et al., 2007;Krzyszkowski & Pijet, 1993;Štěpančíková et al., 2008). A strike-slip SMF sinistral offset (35-40 m) has also been suggested. However, it must be emphasized that the suggested strike-slip component was proposed based only on one truncated stream (Štěpančíková & Stemberk, 2016), even though the neighboring streams are neither offset nor deflected. Likewise, paleoseismological trenches point to displacements by an order of magnitude less (tens of centimeters to a few meters; Štěpančíková et al., 2010). Although we are usually inclined to decrease estimated uplift values, the vertical movements SZCZYGIEŁ ET AL.
10.1029/2020TC006459 21 of 25 along the SFM produced fault ruptures associated with at least 4-5 large earthquakes (M > 6; Štěpančíková et al., 2010;Štěpančíková & Stemberk, 2016). Therefore, it seems that the SFM with its documented paleoseismic history is the most likely seismic source responsible for damage induced in Niedźwiedzia Cave, which is less than 20 km away.

Conclusions
We have examined speleothem damages and passages collapses in Niedźwiedzia Cave, combining geomorphological, geochronological, and seismological research, to better understand factors controlling the nature of the damage. The common multiphase speleothem deformations have been dated with U-series methods, enabling a built-up damage inventory from the latest Pleistocene (∼17 ka) to the late Middle Pleistocene (320 ka). This inventory includes five events: (1) 320-306 ka, (2) 253-236 ka, (3) 162-158 ka, (4) 132-135 ka, and (5) Weichselian, older than 21 ka. Events 1, 3, and 4 are strongly supported by pre and postevent contact laminae dating, whereas events 2 and 5 are less documented and hence remain speculative.
Although we cannot unambiguously exclude other nonseismic agents, such as periodic frost or gravity collapses, compound U-series analysis indicates earthquakes as the most likely trigger mechanism of the major damage in Niedźwiedzia Cave. There are two main arguments for this conclusion: (1) cave damage occurred in both cold and warm periods of the Pleistocene epoch, suggesting climate independence, and (2) not only did the ceiling and walls suffer from collapse, but passage floors were also deformed.
We applied ground motion models and speleothem failure criteria and compared them with the scant records of historical and prehistoric earthquakes to quantify the probable seismic source size, which is most likely the Sudetic Marginal Fault. This fault is located less than 20 km from the cave and caused a documented M6.3 earthquake in the Late Pleistocene and Holocene. The potential of the SMF as the principal seismic source is shown on the maps ( Figure 10) assuming equal probability of epicenter locations along the fault trace. Probable distributions of PGAs caused by magnitudes M5.7, M6.4, M7.1, and M7.6 ( Figure 10) indicate that the magnitude threshold for the failure of majority of speleothems in Niedźwiedzia Cave is M > 6.3. In conclusion, we propose that the SMF can produce PGA amplitudes at a distance between the fault and the cave, forcing the speleothems to break.
Plausible seismogenic sources are faults limiting the UNKG from the east. Although there are no historical data that would help to estimate the seismic hazard herein, the short distance between the cave and faults (from several hundred meters to 8 km) reinforced with an earthquakes of intensity ca. I 0 ∼6.5-7, should suffice to destroy the speleothems. The I 0 ∼6.5-7 intensity has been reported for the HPFZ seismic zone; thus, its eastern extension-the Trzebieszowice-Biela Fault Zone-might also be considered a potential seismic source area.
Our study unravels three essential advantages of speleoseismology. First, broken speleothems and cave collapses may serve as environmental earthquake effects, which can occur with a moderate earthquake if the cave is only a short distance from the epicenter (<20 km). Such moderate-magnitude earthquakes are rarely preserved in geological records. Second, caves shielding recorded deformation from erosion may reveal seismic events, after which no trace is preserved on the surface. Finally, the wide range of U-Th ages that we provide proves the enormous potential of the U-series method to reach far beyond the "classic" geochronometers widely used now in paleoseismology, such as 14 C or OSL.

Data Availability Statement
All the data for sample dating are reported in Table 1. Speleothem physical and mechanical data are reported in Table 3, and raw data as well as calculations used for diagrams in Figures 5 and 6 can also be found in the Zenodo repository at https://doi.10.5281/zenodo.4499062